Add the brown sugar, flour, egg yolks, and salt to a saucepan set over medium heat. Stir in the milk and heavy cream.
1 cup brown sugar, ½ cup all-purpose flour, 4 large egg yolks, 1 cup milk, ½ teaspoon kosher salt, 1 cup heavy cream
Whisk continuously for 3-4 minutes, until the mixture thickens to custard-like consistency. Set aside. (The custard-like mixture should be quite thick. If it's not, keep cooking and whisking and it will eventually thicken up.)
Add the granulated sugar to a separate saucepan or skillet over low-medium heat.
1 cup granulated sugar
Using a wooden spoon, stir the sugar until it melts and caramelizes. Stir in the butter until incorporated.
3 tablespoons unsalted butter
Pour the caramelized sugar into the custard mixture, whisking until combined.
Pour the pie filling into the baked pie crust.
1 9-inch pie crust
Cover with plastic wrap, and place in the refrigerator.
Chill for at least 2 hours (overnight is best) or until the filling is set.
Top with whipped cream.
Whipped cream